An exemplary form of an access network includes a passive optical network (PON) system. Due to a recent increase in data traffic in the access network, a conventional time division multiplexing (TDM) PON system suffers from an insufficient band. Therefore, as described in Patent Literature 1 and Non Patent Literature 1, a time and wavelength division multiplexing (TWDM)-PON system has been examined. Specifically, an optical line terminal (OLT) has a plurality of optical wavelength channels that is used by an optical network unit (ONU), i.e., a slave station device, and the number of ONUs that perform the TDM in a single optical wavelength channel is reduced, whereby transmission capacity per ONU is increased.